1. Field of the Invention
The present invention relates to an aberration correction system (refracting field corrector system) and an astronomical telescope for astronomical observation using the same, and more particularly to an aberration correction system arranged in such a manner that a prime focus corrector, which corrects an aberration of a reflecting mirror (a primary mirror) to widen the visual field of the prime focus, comprises a compound lens which corrects a deviation that occurs due to a difference in the wavelength of light of a star image caused from dispersion by the atmosphere that takes place when an observation is made at a point that is not the zenith. The present invention also relates to an astronomical telescope using the same to have a wide visual field and to be capable of observing a subject at an excellent resolution.
2. Description of the Prior Art
FIG. 1 is a schematic view which illustrates an optical system of a Cassegrain telescope. Referring to FIG. 1, the optical system is arranged in such a manner that a luminous flux from a star or the like in the celestial sphere transmitted in a direction from a left portion of FIG. 1 is reflected and condensed by a mirror 1 composed of a paraboloid and so forth as to be focused on a prime focus 3. Then, the luminous flux is allowed to pass through a secondary mirror 2 having a convex hyperboloid, and then it forms an image of an object (an image of a star) on a plane 4.
Although the Cassegrain telescope is used as a reflector having a large or an intermediate aperture, it encounters a problem that its large off-axis aberration excessively narrows the observation visual field.
On the other hand, a RC (Ritchey-Chretien) telescope is available which enables a wide angle of field to be obtained due to the fact that its structure is arranged in such a manner that the primary and secondary mirrors are each composed of aspheric surfaces near hyperboloids. The optical system of the RC telescope comprises an aplantic lens correcting a spherical aberration and a comatic aberration.
However, the RC telescope also enables a satisfactorily wide visual field to be obtained because it has a large amount of astigmatism. Furthermore, the fact that the primary mirror of the RC telescope has a concave hyperboloid, inhibits excellent focusing on the prime focus 3.
Accordingly, an astronomical observation with a wider visual field is enabled by an arrangement of the RC reflector made as shown in FIG. 2 such that a prime focus corrector system 100 for improving the imaging performance on to the prime focus of the primary mirror 1 is disposed in place of the secondary mirror (omitted from illustration). An observation point except the zenith, encounters a focusing position of light for each color that can be varied due to a difference in the index of refraction (so-called dispersion of the atmosphere) caused by the wavelength of the atmospheric layer surrounding the earth. Therefore the star image is divided into spectra, causing a problem to arise in that the observed image spreads vertically and accordingly the image quality deteriorates.
It might therefore be considered feasible to employ an optical member (an atmospheric dispersion corrector unit) for correcting an atmospheric dispersion in a portion of a prime focus corrector system 100 to prevent the deterioration of the image quality.
Referring to FIG. 2, reference numerals 5, 6 and 11 represent prime focus corrector lenses.
The structure shown in FIG. 2 has a small number of lenses, that is, three lenses in order to prevent generation of ghost and minimize the light quantity loss. Another structure has been considered which comprises a phosphate fluor crown glass to reduce chromatic aberration generated from the corrector system.
Reference numeral 101 represents an optical member for correcting atmosphere dispersion. The optical member 101 is composed of a pair of plane glass plates 101a and 101b each of which is formed by bonding two wedge-shape glass plates 7 and 8 and two wedge-shape glass plates 9 and 10. The glass plates 7 and 10 are crown glass plates, while the glass plates 8 and 9 are flint glass plates.
Reference numeral 12 represents a window member for a sensor such as a CCD for observing a focal plane. When an astronomical observation is performed, direct vision spectral prisms 101a and 101b are rotated around an optical axis in opposite directions in accordance with the amount of the atmosphere dispersion to correct the atmosphere dispersion.
The conventional aberration correction system has been arranged such that the conventional method for correcting the atmosphere dispersion is simply combined with the prime focus corrector system for correcting the aberrations caused from the primary mirror.
As a result, the size and the weight of the prime focus corrector system cannot be reduced, and accordingly a problem arises in that the prime focus corrector system cannot easily be handled when the observation focal position of a multi-purpose large-size telescope is changed over (for example, from Cassegrainian focal point to main focal point) in a usual manner. What is worse, the number of the optical elements increases, causing ghosts to be generated easily and light quantity loss cannot be prevented.
Su Ding-qiang et al. disclosed an optical element capable of correcting aberrations generated due to a primary mirror and chromatic deviation occurring due to atmosphere dispersion (Astron Astrophys., 156 381 (1986) and Astron Astrophys., 232 589 (1990)).
However, the foregoing optical element cannot satisfactorily correct both the aberration of the main mirror and the chromatic deviation occurring due to the atmosphere dispersion while reducing the overall size of the lens system.